1. Field of the Invention
The present invention relates to the preparation of aqueous solutions of olfinically unsaturated hydrophilic monomers, and, more especially, to the preparation of such monomer solutions well adapted for facile direct photopolymerization into water soluble polymeric flocculants.
2. Description of the Prior Art
It is well known to this art to prepare polymeric flocculants via photopolymerization of acrylic monomers, specifically acrylamide, methacrylamide, acrylic or methacrylic acids, optionally the salts thereof, and quaternary ammonium salts derived from dialkylaminoalkyl (meth)acrylates.
It is characteristically required that such organic polymeric flocculants be soluble in water, that they have high molecular weights (thus, high intrinsic viscosities), that they rapidly dissolve in water, and that they leave no insoluble residue following their dissolution.
And insofar as the preparation of such organic polymeric flocculants is concerned, it is desirable that the polymerization be effected continuously, utilizing solutions which are as highly concentrated as possible, but also that the heat generated during polymerization, or exotherm, be quickly dissipated to avoid polymer degradation.
The aforesaid various requirements, together with several others, have culminated in the preparation of flocculating polymers by means of a process whereby an aqueous solution of monomers, containing a photoinitiator, is continuously deposited in the form of a thin layer onto a mobile support, or endless belt, and there continuously exposed to photopolymerizing irradiation, e.g., ultraviolet (UV) radiation.
The preparation of the aqueous monomer solution, prior to irradiation, nonetheless, itself poses certain technical problems. In fact, in addition to the actual dissolution of the monomers, it is necessary to incorporate in said solutions a photoinitiator and, in certain instances, a base or alkaline agent, such as sodium hydroxide, in order to raise the pH of such solutions.
Furthermore, where it is of advantage to irradiate and photopolymerize in the absence of oxygen, the dissolved oxygen must be removed from the monomer solutions (this oxygen has frequently been introduced earlier, and on purpose, in considerable amounts, in order to inhibit polymerization during storage).
Thus, if the preparation of such monomer solutions is effected without specific precautions or by inadequate methods, various detrimental consequences may be experienced; monomers that are sensitive to the action of alkaline agents may begin to saponify, which ultimately results in flocculating polymers of inconsistent or poor quality; inadequately deoxygenated solutions may be difficult to polymerize; finally and most importantly, there is a contrary risk of premature and "accidental" polymerization within the supply apparatus itself, such polymerization being increasingly dangerous the more advanced the stage thereof; premature and accidental polymerization is even more detrimental, because the equipment is selected and arranged for continuous operation, and there exists, therefore, a risk of the entire installation being blocked, particularly by clogging of the feed and other lines; such premature and accidental polymerization can only be remedied by a shutdown in operation (after several days of production), and thus it is critically necessary to exclude from such solutions any active agents that may initiate or favor any premature polymerization, whether slight or extensive.
Heretofore, the principal means employed for the preparation of the aqueous monomer solutions have been either the direct mixing of the constituents of the solutions, or the utilization of a series of mixers.
However, the direct mixing technique is not suitable when the aqueous monomer solution has been prepared beforehand and the solution then oxygenated in order to enable storage without premature polymerization.
The use of a plurality of mixers, in series, in spite of the advantages thereof (perfect admixture or one reagent being added to a successive reagent), nonetheless, does have its own disadvantages, in particular, the excessive size of the apparatus required; the overly lengthy retention time of the reagents; and the utilization of agitators, stirrers, or any other system of mobile metallic elements is again not feasible, because same may favor the aforediscussed premature polymerization.